The present invention relates to an internal combustion engine, comprising a cylinder block with at least one cylinder chamber, a piston movable in the cylinder chamber, a cylinder head, and, in the cylinder head at least one exhaust channel having an openable and closable outlet from the cylinder chamber and at least one inlet channel having an openable and closable inlet valve to the cylinder chamber, said inlet forming an angle relative to the direction of movement of the piston, at least one spark plug protruding into the cylinder chamber, and a fuel injector, arranged to inject fuel directly into the cylinder chamber in a direction oblique to the direction of movement of the piston, said piston having a top with a surface portion forming an angle with a plane normal to the direction of movement of the piston and connected to a bowl-shaped cavity, which, together with said surface portion, is disposed to impart air flowing into the cylinder chamber a vortical movement about an axis perpendicular to the direction of movement of the piston.
The motor vehicle industry has of late put more and more effort into developing spark ignition internal combustion engines with direct injection systems, i.e. systems which inject the fuel directly into the cylinder chamber instead of into the inlet manifold. It is a known fact that by injecting the fuel directly into the combustion chamber and stratifying the fuel-air mixture, so that a non-homogeneous mixture with a concentration of the fuel in the area around the spark plug is obtained, it is possible to have the engine operate with a very lean mixture of air and fuel. This is utilized in the latest generation of gasoline engines with fuel injection during engine operation at low partial loads, when the fuel/air ratio can be kept as low as 1:35, to be compared with a fuel/air ratio of 1:14 in a conventional engine without direct injection working with an homogeneous fuel-air mixture over its entire load and rpm range. By operating the engine with this lean mixture at low partial loads, the total fuel consumption of the engine can be substantially reduced thus reducing, operating costs and environmental impact.
A number of solutions have been suggested to solve the problem of achieving this stratified fuel-air mixture, which provides a concentration of fuel in the area about the spark plug. Common to all of them is that a vortex is generated in the inlet air and that the fuel is injected into the combustion chamber in such a manner in relation to the air vortex that the fuel is transported up to the area about the spark plug, so that the concentration of fuel in the fuel-air mixture will be higher in the vicinity of the spark plug than in the peripheral area of the combustion chamber.
It is known to generate the vortex either about an axis perpendicular to the direction of movement of the piston, as in the combustion engine described by way of introduction, or about an axis in the direction of movement of the piston. The first type of vortex is usually named xe2x80x9ctumblexe2x80x9d and the second is named xe2x80x9cswirlxe2x80x9d. The vortex about the horizontal axis is obtained by interaction between the valve angle, i.e. the angle of incidence of the inlet channel, the shape of the channel and the shape of the top of the piston. With varying valve angles, piston top shapes and injection angles, various results are obtained with regard to stratification of the fuel-air mixture at low partial loads, but other factors as well, such as the level of turbulence at the moment of ignition in the homogeneous fuel-air mixture at higher partial loads and full load are affected. These factors in turn affect the increase in efficiency which can be obtained in combination with the operating quality of the engine and the exhaust emissions compared to an engine without direct injection, which always works with a homogeneous fuel-air mixture.
The purpose of the present invention is in general, starting from a known internal combustion engine of the type described by way of introduction, to achieve an engine which has a higher efficiency than what has hitherto been achievable, at the same time as the running quality is high and its exhaust emissions are low.
This is achieved according to the invention by virtue of the fact that the angle between said surface portion and said plane normal to the direction of movement of the piston, and the angle between the mouth of the inlet channel and the direction of movement of the piston are so selected in relation to each other and to the direction of movement of the piston that, during predetermined operating conditions, the inlet air flowing into the cylinder chamber outside the piston cavity is imparted a primary vortical movement which generates a secondary vortical movement in the opposite direction in the piston cavity.
It has been shown that it is thereby possible to obtain a gas movement, which has a suitable combination of injection characteristic and injection timing to provide a suitable fuel-air mixture at the spark plug at the point of ignition and a sufficient level of turbulence in the combustion chamber during stratified operation (low partial loads). Furthermore, there is obtained during homogeneous operation (higher partial load and full load) a homogeneous fuel-air mixture with sufficient level of turbulence at the moment of ignition.
Tests performed have shown that the above specified gas movement is obtained if the valve angle (the angle of incidence of the inlet channel) is greater than 20xc2x0, preferably at least 25xc2x0 and at most 30xc2x0, if the angle between said surface portion and a plane normal to the direction of movement of the piston is equal to or less than the valve angle but preferably not less than 75% of the valve angle, and if the height of the piston cavity is at least 10% but at most 25% of the diameter of the cylinder.